display 'on another call' on ip phones

[achilles1]

I must configure a way that when phone A calls phone B (internal call) and phone B is busy, the screen would display: On another call. Party B has callwaiting, so will see an incoming call.

There are only Polycom IP331 phones in the network and every sip account in asterisk has callwaiting activated (as per client demand).

Any help is greatly appreciated.